I agree with the comments about the hardware in the older AppleTV--it plays iTunes content very well. But when I've used some of the hacks it really struggles with other stuff--the exception being DVDs that are ripped to VIDEO_TS folders. It can handle that with ease, but then again, that is standard def and plenty of old Macs could handle that.
As a current AppleTV owner, I'm more curious if the 99 cent rentals will be available to me. Once other studios/networks get on board, I think that will be a nice option for people that miss a show or hear about a show and want to check it out at their convenience.